1. Field of the Invention
The present invention relates to thermal transfer printing, and more particularly, to an apparatus for braking a rotating print media roll in between printing operations.
2. Description of Related Art
In the field of bar code symbology, vertical bars of varying thicknesses and spacing are used to convey information, such as an identification of the object to which the bar code is affixed. The bar codes are typically printed onto paper substrate labels having an adhesive backing layer that enables the labels to be affixed to objects to be identified. To read the bar code, the bars and spaces are scanned by a light source, such as a laser. Since the bars and spaces have differing light reflective characteristics, the information contained in the bar code can be read by interpreting the laser light that reflects from the bar code. In order to accurately read the bar code, it is thus essential that the bar code be printed in a high quality manner, without any streaking or blurring of the bar code. At the same time, it is essential that the adhesive backing layer of the labels not be damaged by heat generated during the printing process.
In view of these demanding printing requirements, bar codes are often printed using thermal transfer printing techniques. In thermal transfer printing, a label sheet of the print media is drawn between a platen and a thermal print head. The label sheet may be impregnated with a thermally active chemical, or alternatively, a thermally active ink ribbon may be drawn along with the label sheet between the platen and the thermal print head. The thermal print head has linearly disposed printing elements that extend across a width dimension of the label sheet. The printing elements are individually activated in accordance with instructions from a controller. As each printing element is activated, the thermally active chemical activates at the location of the particular printing element to produce the printed area. The label sheet is continuously drawn through the region between the platen and the thermal print head, and in so doing, the bar code is printed onto the label as it passes through the region. Other images, such as text characters, can be printed in the same manner.
The thermal transfer printer includes a mechanism for transporting the label sheet from a supply hub to the print region. It is desirable within the art to increase the rate at which the labels are printed. At the same time, it is also desirable to increase the overall width of the label (e.g., up to seven inches), which has driven a substantial increase in weight of the label roll (e.g., up to ten pounds). As a result, it is necessary to accelerate the heavy roll from a stationary state to an operational speed (e.g., greater than ten inches per second) as quickly as possible to prevent printing pitch errors. To overcome this problem, a low friction roller that permits a high acceleration rate is typically used to carry the label roll. A drawback of the conventional low friction rollers is that they make the rotating label roll difficult to decelerate and stop from the operational rate once the printing operation has completed. If the roll is permitted to decelerate on its own, a large quantity of the print media would spool off the roll, causing print registration errors, label jams, and consequent waste of the print media.
Accordingly, it would be desirable to provide a braking mechanism for a low friction print media supply hub for use in a thermal transfer printer. Such a braking mechanism should enable rapid acceleration at the commencement of printing operations and rapid deceleration once the printing operations have completed.